12 DISCOVERY REPORTS 



deep water at lower latitudes and a high content in the Antarctic bottom water south 

 of 56 S, maximum phosphate occurs in these two localities ; the first lies between the 

 Antarctic convergence and the Atlantic-Indian cross-ridge, and the second at about 

 3 8° S, although the significance of the latter position may be affected by the fact that 

 the section is composed of two sets of observations over a year apart and the division 

 lies close to 38 S, i.e. annual variation is possibly the cause of the higher values at 

 St. 1567 at 1500-2000 m. Also it is not yet certain how far south the last traces of North 

 Indian deep water extend in the upper layers of the warm deep water which is mainly 

 of North Atlantic origin south of 20 S (Clowes and Deacon, 1935). The south-going 

 warm deep water rises steeply towards the surface on the northern side of the Atlantic- 

 Indian cross-ridge and continues to rise to the Antarctic convergence. Its upper layers 

 are in contact with the descending Antarctic surface water, and on the southern side 

 of the cross-ridge considerable mortality and decomposition of plankton must occur to 

 account for the high values of phosphate content found there. The regenerated 

 phosphate is then swept to the far south on the upper layers of the warm deep water, 

 so that at the ice-edge in 64I S the content immediately below the discontinuity between 

 the surface and warm deep currents is 156 mg. 



The source of the high phosphate values in the upper layers of the North Indian deep 

 current is at present unknown, and in any event these high values should be checked 

 by other observations. 



In the South Pacific Ocean we have only data from the western part between New 

 Zealand and the ice-edge north of the Ross Sea. One section (12, Plate XVII) was made 

 in late winter, September 1932, and the other section (13, Plate XIX) in midsummer, 

 January 1934. Phosphate leaves the Antarctic zone in the South Pacific in two currents : 

 (1) in the Antarctic surface water which sinks at the Antarctic convergence and spreads 

 northwards as part of the Antarctic intermediate current, and (2) in the highly saline 

 bottom water which flows northwards from the far south. The return of phosphate to 

 the south is effected in the mixing processes which take place between the Antarctic 

 intermediate current and the bottom current with the warm deep water. The largest 

 concentrations of phosphate are found in the bottom layers of the intermediate water 

 and the upper layers of the warm deep water. In the South Pacific sector the Antarctic 

 intermediate water is a very strong current, and the boundary between this current and 

 the warm deep water is a thick layer in which maximum decomposition of the plankton 

 is occurring, thus releasing the phosphate which is carried back to the south in the 

 upper layers of the warm deep water. However, as is stated on p. 15 of this report, not 

 all the water in the upper part of the warm deep water reaches the Antarctic zone. 



The silicate cycle in the eastern part of the South Atlantic Ocean can be followed by 

 reference to Plate VI. This shows a section from the ice-edge in 69 21' S, 9 33-8' E 

 northwards to 3 46-2' N, 12 55-1' W, although north of i2h° S the observations only 

 extend to 1400 m., i.e. just below the intermediate water. Silicate may be seen to leave 

 the Antarctic zone in two ways ; the Antarctic surface water sinks at the Antarctic con- 

 vergence carrying with it silicate up to about 2500 mg. in a free form plus an unknown 



